Conventionally, there has been known, as a recording means for a camera, a recording method which includes forming on a surface of an imager an object image by the imager using a photoelectric converter such as a CCD (Charge Coupled Device) or CMOS (Complementary Metal-Oxide Semiconductor), and converting an amount of light of the object image into electric output by the photoelectric converter.
According to technological progress on fine processing technology in recent years, a Central Processing Unit (CPU) has been speeded up and a recording medium has been highly integrated, so that an extra-large volume of image data, which could not be handled before, can be processed at high speed. In addition, a light-receiving element has been also highly integrated and reduced in size. The high integration allows higher spacial frequency to be recorded, and the reduction in size allows downsizing of the whole camera.
However, the high integration and reduction in size reduces a light reception area of the photoelectric converter. Accordingly, influence of noise may be increased caused by decrease in the electric output. To avoid this, there has been attempted that the amount of light reaching the light-receiving element is increased by increasing an aperture ratio of an optical system, or that an extra-small lens element (i.e., micro-lens array) is disposed in front of the element. The micro-lens array guides light beam incident on a gap between the adjacent elements onto the element, however, it restricts the position of an exit pupil of the lens system. In particular, when the position of the exit pupil of the lens system comes close to the light-receiving element, an angle formed by chief ray reaching the light-receiving element and by the optical axis is increased, and consequently, an angle formed by off-axis light beam heading to the periphery of a screen and by the optical axis is more increased. Accordingly, the off-axis light beam would not reach the light-receiving element, thereby providing an insufficient amount of light.
There is known, for instance, a positive-negative-positive-positive four-group zoom lens as the zoom lens appropriate for the video camera, digital still camera, or the like, which records the object image by the above-mentioned photoelectric converter.
The positive-negative-positive-positive four-group zoom lens includes a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power, sequentially arranged from an object side. When a positional state of the lenses is varied from a wide-angle-end state to a telephoto-end state, the first lens group and the third lens group are fixed along the optical axis, the second lens group is moved to an image side for varying magnification, and the fourth lens group is moved for compensating a variation in a position of an image plane caused by the movement of the second lens group.
For instance, there have been known zoom lenses disclosed in Japanese Unexamined Patent Application Publication Nos. 6-337353 and 2002-365544.
However, according to the above-mentioned conventional zoom lens, when a zoom ratio (=focal length in the telephoto-end state/focal length in the wide-angle-end state) is increased, an amount of the movement of the second lens group is increased. Consequently, the off-axis light beam passing through the first lens group comes away from the optical axis, and lens diameters of the first lens group may be increased.
Particularly in the conventional positive-negative-positive-positive four-group zoom lens, the amount of the movement of the second lens group for magnification-varying operation is increased in order to increase the zoom ratio. Therefore the off-axis light beam passing through the first lens group likely comes away from the optical axis, thus being difficult to achieve both the reduction in lens diameters and the high variable magnification.
To solve the above-described problems, the present invention provides a zoom lens having a high zoom ratio and easily allowing lens diameters to be reduced, and an imaging apparatus provided with the zoom lens.